The presence of a comorbid depressive disorder increases risk for morbidity and mortality in patients with coronary heart disease. Serotonergic dysfunction is thought to contribute to mood disorders and is also a risk factor for heart disease, raising the possibility that altered 5-HT homeostasis contribute to comorbidity between neuropsychiatric and cardiovascular disorders. Recently, we discovered a physical and functional interaction between 5-HT transporter (SERT) and integrin ?3 in platelets. While integrin ?3 is essential for platelet function, SERT plays a prominent role in the modulation of neuronal 5-HT signaling. The integrin ?3 polymorphism Leu33Pro is associated with cardiovascular disease due to enhanced integrin signaling and platelet aggregation. This ?3 variant is also associated with hyperserotonemia in autistic patients, due to its upregulation of SERT surface expression and activity. Our preliminary studies indicate that integrin ?3 is expressed presynaptically (as the dimer ?V?3), where it physically associates with SERT. Additional studies suggest that manipulation of ?V?3 signaling directly influences SERT function. These findings lead to my hypothesis that the SERT/ ?V?3 complex represents a critical and conserved facet of synaptic SERT regulation, mimicking its actions in platelets. This proposal seeks 1) to elucidate the physical basis by which ?V?3 influences SERT, 2) to establish the contribution of ?V?3-based activation to SERT regulation and 3) to understand the impact of Leu33Pro ?3 coding variation on SERT function. In Aim 1, we focus on mapping the ?3 binding domain in SERT utilizing biochemical and in vitro approaches, providing a route to the development of small peptides that can disrupt SERT/ ?V?3 interactions ex vivo. In Aim 2, we utilize small molecule ?V?3 ligands to delineate the influence of ?V?3 activation on SERT function ex vivo, as well as 5-HT clearance in vivo. In Aim 3, we determine the effects of the Leu33Pro ?3 variant on 5-HT levels and SERT activity in both platelets and in the brain. We hypothesize that the expression of integrin Pro33?3 will lead to constitutively- elevated SERT function in platelets and brain. Together, these studies represent the first opportunity to examine the presynaptic impact of ?V?3 and help elucidate pathways supporting comorbidity between mental illness and cardiovascular disease.